CHANGELOG: update for v0.26.4

CHANGELOG.md

+v0.26.4
+-------
+
+This is a security release fixing insufficient validation of submodule names
+(CVE-2018-11235, reported by Etienne Stalmans) and disallows `.gitmodules` files
+as symlinks.
+
+While submodule names come from the untrusted ".gitmodules" file, we blindly
+append the name to "$GIT_DIR/modules" to construct the final path of the
+submodule repository. In case the name contains e.g. "../", an adversary would
+be able to escape your repository and write data at arbitrary paths. In
+accordance with git, we now enforce some rules for submodule names which will
+cause libgit2 to ignore these malicious names.
+
+Adding a symlink as `.gitmodules` into the index from the workdir or checking
+out such files is not allowed as this can make a Git implementation write
+outside of the repository and bypass the `fsck` checks for CVE-2018-11235.
+
+libgit2 is not susceptible to CVE-2018-11233.
+
 v0.26.3
 -------